Optical scanners operate by imaging an object (e.g. a document) with a light source, and sensing a resultant light signal with an optical sensor array. Each optical sensor or photoreceptor in the array (typically a linear array) generates a data signal representative of the intensity of light impinged thereon for a corresponding portion of the imaged object. The data signals from the array of sensors are then processed (typically digitized) and stored in a temporary memory such as a semiconductor memory or on a hard disk of a computer, for example, for subsequent manipulation and printing or display, such as on a computer monitor. The image of the scanned object is projected onto the optical photo sensor array incrementally by use of a moving scan line. The moving scan line is produced either by moving the document with respect to scan bar assembly that includes the array of optical sensors, or by moving the scan bar assembly relative to the document. Either or both of these methods may be embodied in a flat bed scanner, multi-function printer, or any scanner having manual and automatic feed capabilities.
A common type of scanner uses a contact image sensor (CIS) scan bar. A CIS scan bar includes a contact image sensor scan element having a length that is substantially equal to the width of the scanning region. The photoreceptors in a CIS are substantially the same size as the pixel resolution of the scanner. The CIS has a short depth of field and is typically mounted beneath the transparent plate (scanner glass) upon which the document is placed. A scan bar assembly includes the CIS scan element, as well as gears for power transmission to move the scan bar assembly. One or more roller spacers in the CIS scan bar assembly are biased against the bottom of the scanner glass so that the CIS scan element is always at substantially the same distance from the top of the scanner glass.
U.S. Pat. No. 6,246,492 discloses a movable module, which includes a contact image sensor and a driving motor and which can slide back and forth along a track to scan an image. The driving motor exerts a force by means of a pinion on a fixed rack attached to the frame of the scanner.
U.S. Patent Application Publication 2009/0034019 describes a scanner module including the optical components, where the scanner module is carried by a carriage that includes a motor and associated gears. FIG. 1 (prior art) is a copy of FIG. 2 of U.S. Patent Application Publication 2009/0034019 and FIG. 2 (prior art) is a copy of FIG. 4 of U.S. Patent Application Publication 2009/0034019. Scanner 120 includes platen 122, carriage 124, wheels 126, bias 128, drive 130, light source 132, reflected light capture unit 134, and sensor array (not shown). Light source 132, reflected light capture unit 134 and the sensor array are joined to one another to form a scanner module 135 which includes a body 204 and wheels 126. Module 135 is carried by carriage 124. Platen 122 includes a plate, at least a portion of which is transparent, configured to support on its top surface 144 a document or other article to be scanned. Central portion 200 comprises that portion of platen 122 through which light is transmitted and through which reflected light passes. Side portions 202 may be transparent or may be opaque. Side portions 202 provide surfaces against which wheels 126 rotate. Carriage 124 carries reflected light gathering unit 134, light source 132 and the sensor array as they are moved across and along platen 122. Scan module 135 includes body 204 and two opposing wheel wells 206 that are sized to receive wheels 126, which are retained by caps 210. Bias 128 includes one or more members, such as wheels 214, configured to resiliently urge carriage 124, wheels 126 and reflected light gathering unit 134 towards platen 122. As a result, wheels 126 are maintained in constant contact with surface 152 as carriage 124 is moved across platen 122. Wheels 214 are urged against a stationary surface 216 (schematically shown) associated with the housing of scanner system 120. Drive 130 is configured to move carriage 124 in either direction as indicated by arrows 158 (called the scan direction herein). Drive 130 moves carriage 124 and reflected light gathering unit 134 across platen 122 such that a document may be scanned. In the example shown in FIGS. 1 and 2, drive 130 includes motor 220, worm gear 224, drive gear 225, pinion gears 226, 228 and rack 230 (schematically shown in FIG. 1). Motor 220 is carried by carriage 124 and is connected to an encoder (not shown). Worm gear 224 is in engagement with drive gear 225 which is part of a compound gear also including pinion gear 226. Pinion gears 226 and 228 are in engagement with rack 230. Rotation of pinion gears 226, 228 results in carriage 124 being driven along rack 230 relative to platen 122. Because scanner module 135 (FIG. 2) is made separately from carriage 124 (FIG. 1), some means of affixing scanner module 135 to carriage 124 is required. Visible in FIG. 1, but not originally labeled in U.S. Patent Application Publication 2009/0034019 are bolts 125 for attaching scanner module 135 to carriage 124. Also originally unlabeled in FIG. 1 of U.S. Patent Application Publication 2009/0034019 (but identifiable by one who is familiar with conventional scan bar designs) is gear retainer tab 127, which is typically formed of a piece of stamped metal that is bolted to carriage 124 with one of the bolts 125. A further component that is unlabeled in U.S. Patent Application Publication 2009/0034019 but that is readily identifiable in FIG. 1 is motor printed circuit board 221 that is used for connecting power to motor 220 and for attaching the rotary encoder sensor that monitors rotation of the motor axle.
The prior art scan bar assembly shown in FIGS. 1 and 2 is satisfactory in many applications. However, for small footprint scanners or multi-function printers, a limitation to the reduction in overall size can be the size of the scan bar assembly. In particular, a typical width of a scan bar assembly along a scan direction 158 (approximately the distance between outer edges of wheels 214 in FIG. 1) is about 7 centimeters or greater. A smaller footprint multi-function printer can result in improved convenience to the user, as well as cost savings. In addition, in the prior art configuration of FIGS. 1 and 2, the heaviest component of scan bar assembly (motor 220) is positioned relatively far from the light gathering unit 134 and the associated scan element, as well as from the drive bearing datums. Such a configuration can lead to motion instability and vibration effects. Improved motion stability allows faster scanning, through reduced settling time, and also requires less mechanical structure support.
What is needed is a scan bar assembly having a more compact configuration, and having the center of mass of the scan motor moved closer to the centerline of the scan element and the drive bearing datums, in order to achieve lower cost, improved motion stability, and faster scanning capability.